Piston-cylinder units are known which, with the aims of making the reciprocal position between a cylinder and the respective piston translatable therein recognizable, equip the stem solidly constrained to the piston, for example made of steel, with one or more reference marks that are detectable by suitable optical sensors provided on the cylinder. In greater detail, each reference mark is typically obtained on the surface of the stem of the piston, for example by means of laser machining, defining a zone of a dark color (for example black) which contrasts with the rest of the surface of the stem, typically metallic and light-colored.
A much-lamented problem for the piston-cylinder units lies in the fact that the reference mark, with the use of the piston, for example by mechanical wear, tends to lighten, and in particular tends to reduce the chromatic contrast existing between the reference mark and the surrounding surface of the stem, effectively making the reference mark illegible to the sensor, as the contrast is less than the minimum threshold of detectability of the sensor, which thus will no longer find the position established as the reference mark (for example the centered position of the piston with respect to the cylinder).
A method and a piston-cylinder unit of known type are described in international patent application no. WO 2014 118709. The method and the piston-cylinder unit described in the above-described patent application propose to resolve the above-cited drawback making the minimum threshold of the sensor variable according to wear. In practice, the patent application proposes progressively varying the detection threshold of the sensor as the reference mark progressively loses color.
However, this solution leads to the onset of further problems, among which the fact that by continually varying the detection threshold of the sensor, the sensor will detect, as the established position, any point of the stem of the piston that has a contrast with the progressively lighter surface of the stem, i.e. equal to the updated threshold value.
In practice this solution increases the risk of identification of false positives, i.e. signaling of positions which should correspond to the established reference position, but which in reality vary therefrom.
For example, a wrong positioning signal of the piston of the centered position thereof might occur even in very different positions from the effective centered position.
This means, for example in the case of a piston-cylinder unit adopted for guiding steering axes, that the blocking of the steering can be enabled in a false centered position, in which for example the wheels are not straight but steered, with consequent predictable drawbacks of poor maneuverability and poor vehicle safety.
An aim of the present invention is to obviate the above-mentioned drawbacks of the prior art, with a solution that is simple, rational and relatively inexpensive.
The aims are attained by the characteristics of the invention as reported in the independent claim. The dependent claims delineate preferred and/or particularly advantageous aspects of the invention.